1. Field of the Invention
The present invention relates to a laser gauge interferometer for measuring a displacement of a moving member at high precision.
2. Description of the Related Art
Up to now, a laser gauge interferometer has been known in which a measurement interferometer for generating a measurement output based on a displacement of a moving member which is an object to be measured and a correction interferometer for generating a measurement output based on a change in refractive index of air are combined to perform measurement while the influence of the change in refractive index of air is corrected (see Japanese Patent Application Laid-Open No. H02-001501). In the technology described in Japanese Patent Application Laid-Open No. H02-001501, a single interferometer serves as both the measurement interferometer and the correction interferometer. The interferometer is set on the moving member and moved together with the moving member between a pair of mirrors. A laser beam from a laser oscillator is divided into two by a beam splitter of the interferometer to generate a measurement laser beam (laser beam for measurement) for the measurement interferometer and a correction laser beam (laser beam for correction) for the correction interferometer, to thereby form respective optical paths without overlapping with each other. In order to equally influence both the laser beams by the change in refractive index of air, the optical paths of both the laser beams are formed close to each other.
In the measurement interferometer, the measurement laser beam is emitted to only one of the pair of mirrors. In contrast to this, in the correction interferometer, the correction laser beam is emitted to both the mirrors. Therefore, the optical paths of the respective laser beams which are formed between the one of the mirror and the interferometer are close to each other, but an optical path through which the measurement laser beam does not pass and only the correction laser beam passes is formed between the other mirror and the interferometer. A distance between the mirrors is constant irrespective of the movement of the moving member. Thus, the change in refractive index of air may be obtained based on a change in optical path length which is measured by the correction interferometer, and the obtained result may be reflected in a result obtained by measurement by the measurement interferometer.
However, in recent years, the further improvement of positioning precision of the moving member has been desired. In the conventional structure described above, a measurement error is large. To be specific, the optical path of the measurement laser beam and the optical path of the correction laser beam are close to, but separated from, each other. Therefore, a result obtained by correcting the displacement of the moving member has an error because of a spatial difference of the change in refractive index of air. That is, there is a spatial refractive index distribution in air and the changes of the refractive index of air on the respective optical paths are different from each other. Thus, when the result obtained by measurement by the measurement interferometer is corrected based on the result obtained by measurement by the correction interferometer reflecting a different refractive index change, a measurement result of the displacement of the moving member may have an error.
The correction laser beam passes through not only an optical path close to the optical path of the measurement laser beam but also an optical path different from the optical path close to the optical path of the measurement laser beam, and hence the measurement result of the displacement of the moving member may have an increased error.